My Australian fiancée is coming to the UK under the Youth Mobility Scheme. She had to say she wasn't coming for purpose of marriage. Which she wasn't. But since then, we've got engaged

Now we want to marry in the UK, before we settle in Australia. No Certificate of Approval needed - Church of England wedding.
So, how does she handle this at the UK border if she's asked?
She could admit that she's getting married. If we assure them that we're settling overseas after marriage, yet we have no evidence for that yet (other than a bit of research), are they going to take us seriously?
Or, she could deny it? How would they know any different anyways?
Feeling very conflicted about this... so would love advice from anyone caught in the same position. I wish I could just trust the agency to make the right decision when faced with honesty...